A. Field of the Invention
This invention relates to high density polyethylene yarns having a tenacity of at least 12 grams per denier (gpd) and to a process for producing the same at commercially attractive spinning speeds. The term "spinning speed" as used herein means the velocity in meters per minute (m/min.) of freshly extruded fibers, that is, the velocity of the solidified molten polymer in fiber form before it is drawn. The polyethylenes from which the fibers are extruded are high density polyethylenes having a number average molecular weight (M.sub.n) of at least 20,000 and a weight average molecular weight (M.sub.w) less than 125,000. The term "high density polyethylene" is used herein in accordance with conventional terminology and means substantially linear polyethylene having a density of from 0.92 to 1.0 g/cm.sup.3. The term "fiber" as used herein means a single filament or a yarn, that is, a bundle of filaments.
B. Description of the Prior Art
Polypropylene, nylon 6 and nylon 66 fibers are widely used in industry as cordage, for example, rope. Industrial cordage fibers normally have a tenacity ranging from about 6 to about 10 gpd and are commonly referred to as high tenacity fibers. There has been a continuing demand in the cordage industry to provide a lower cost high tenacity fiber suitable for cordage end uses. It is envisioned that such a fiber must meet three criteria in order to be competitive with the fibers presently used as cordage. First, the fiber must be produced from a fiber-forming material which is less expensive than polypropylene, nylon 6 or nylon 66. Secondly, the fiber must have a tenacity at least as high as the fibers presently used as industrial cordage and, preferably, a tenacity of at least 12 gpd. Lastly, the fiber must be capable of being produced at reasonably attractive spinning speeds. While it is generally recognized that polyethylene meets the first criterion, heretofore, it has not been possible to produce polyethylene fibers having a tenacity of at least 12 gpd at reasonable spinning speeds.
Most of the work that has been done to date and reported in the literature on polyethylene fibers is directed to processes for producing high modulus polyethylene fibers as a low cost substitute for glass and graphite fibers, which fibers have been traditionally used as the reinforcing material in composites. Such a process is described in U.S. Pat. No. 3,962,205 and West German Patent 2,650,747. The process involves extruding a high density polyethylene of a specified weight average molecular weight (i.e. M.sub.w) and number average molecular weight (i.e. M.sub.n) to form fibers which are cooled to a temperature of from 100.degree. to 120.degree. C. at the rate of from 1.degree. to 15.degree. C. per minute and then rapidly cooled. The fiber is then drawn at a temperature at least 40.degree. C. below its melting point at a draw ratio of at least 18. This process however apparently must be operated at very slow spinning speeds due in part at least to the the slow cooling step, for example, in the patents a spinning speed of only about 4-5 meters per minute (m/min.) is illustrated. Moreover, applicants have been unable to produce high tenacity fibers (i.e. yarns having a tenacity of 12 gpd or higher) from the particular polyethylenes specified in the U.S. patent even under conditions which would normally maximize tenacity. The German patent differs from the U.S. patent in that it extends the useful polyethylenes to include those having a M.sub.w greater than 200,000 (e.g. 310,000 to 1,000,000), whereas the U.S. patent specified only those polyethylenes having a M.sub.w less than 200,000 and a M.sub.n less than 20,000. However, since the melt viscosity of a polyethylene is directly proportional to its M.sub.w, fibers of the high M.sub.w polyethylenes disclosed in the German patent cannot be produced at commercially feasible spinning speeds by presently known means.
Therefore, it is an object of the present invention to provide polyethylene yarns having a tenacity of at least 12 gpd and a process for producing the same at commercially feasible spinning speeds.
Other objects and advantages of the invention will become apparent from the following detailed description thereof.